Recent research by Menaker and coworkers on the house sparrow has led to the hypothesis that the pineal organ is the "master clock" underlying circadian rhythms in birds. Most striking is their finding that pinealectomy destroys the capacity for free-running circadian rhythmicity. Certain other paradoxical consequences of pinealectomy in the sparrow, however, encourage the alternative interpretation, that secretory products of the pineal may provide only a component of "permissive conditions" for a pacemaker system, located elsewhere. This alternative presumes that pinealectomy is equivalent to an increase in the apparent level of light. The sparrow's rhythmic system is unusually vulnerable to continuous light. This proposal is for a study of circadian rhythms of another carefully chosen passerine bird, the house finch, known to be less sensitive to light. The prediction is that in this species pinealectomy will not produce arrhythmia, but instead only shorten the period of the free-running rhythm, as would moderately increased light intensity. The locomotor activity of twenty adult house finches will be monitored for intervals of several weeks, under a program of light cycles and continuous very dim light, both before and after pinealectomy. Should the initial prediction fail (no free-running rhythms after pinealectomy), a systematic program of chronic melatonin implants into pinealectomized house finches will be undertaken, based on the more general proposition that the pineal provides a necessary tonic influence on a non-pineal pacemaker, but does not provide the critical phasic driving rhythm.